1. Field of the Invention
The present invention relates to a semiconductor light emitting device such as an LED (light emitting diode) and particularly to a technique for improving light extraction efficiency.
2. Description of the Related Art
For LEDs formed of AlGaInP-based material, the band gap of the light emitting layer is larger than the band gap of a GaAs substrate used in crystal-growth. Hence, part of light directed toward the light-extraction-surface side out of the light emitted from the light emitting layer can be extracted, while light directed to the GaAs-substrate side is then absorbed by the GaAs substrate.
Japanese Patent Kokai No. 2002-217450 (Patent Literature 1) discloses an LED which is produced by forming a semiconductor film made of AlGaInP-based material on a GaAs substrate, then sticking the semiconductor film to a support substrate via a reflecting film made of metal having high reflectance, and then removing the GaAs growth substrate. With the LED of this configuration, light directed toward the opposite side from the light extraction surface is reflected by the reflecting film, not absorbed by the GaAs substrate, thus improving the light extraction efficiency of the LED.
However, light incident at an angle greater than or equal to the critical angle on the interface between the semiconductor film and an ambient medium such as air or resin is totally reflected and cannot be extracted to the outside. Light which could not be extracted outside is repeatedly reflected inside the semiconductor film (multiple reflection). The intensity of light propagating inside the semiconductor film decreases exponentially with propagation distance (optical path length). That is, light being multi-reflected inside the semiconductor film is absorbed by the semiconductor film (self-absorption) and is difficult to be extracted outside. For example, if an AlGaInP-based semiconductor film having a refractive index of 3.3 is enclosed in resin having a refractive index of 1.5, then the critical angle is 27° with the reflectance at the interface between the semiconductor film and the resin being about 15%, and thus the proportion of light which can be extracted outside is limited to about 4.5%.
Japanese Patent Kokai No. 2008-103627 (Patent Literature 2) discloses a semiconductor light emitting device having recesses/protrusions formed in the light extraction surface of the semiconductor film. With this configuration, light directed to the light extraction surface is then scattered and diffracted by the recesses/protrusions, thereby reducing the amount of light totally reflected at the interface between the light extraction surface and the ambient medium, and thus the light extraction efficiency can be improved. Further, Patent Literature 2 describes that the recesses/protrusions are formed without making the thickness of the semiconductor film smaller. This is because the smaller thickness of the semiconductor film results in larger series resistance and also insufficient spread of current. That is, insufficient current spread in the semiconductor film results in the occurrence of a region having high current density. When the current density in the semiconductor film exceeds a certain level, an overflow of carriers injected into the light emitting layer occurs, resulting in a decrease in the quantity of carriers that can contribute to light emission, thus reducing luminous efficiency. Thus, current needs to be made to widely spread across in the semiconductor film by securing the thickness of the semiconductor film to a certain degree. Like techniques are disclosed in Japanese Patent No. 4230219 (Patent Literature 3) and Japanese Patent Kokai No. 2003-258296 (Patent Literature 4).